This invention relates to an optical pickup which records a signal in an optical data recording medium and/or reproduces the recorded signal.
2. Description of the Related Art
A conventional optical pickup is assembled by use of individual parts such as an objective lens, a prism, a semiconductor laser, an optical detector, and the like. In such a conventional optical pickup, the lower limitation of the respective parts is determined in terms of an operation in assembling and accuracy of positioning. There is needed a mechanism for adjusting mutual positioning. As a result, there is a disadvantage in the prior art that the size of the optical pickup cannot be made very small.
In order to solve the above problem, there has been proposed a technique in which the optical pickup is integrated by use of an optical waveguide and a grating coupler. For example, there is known an integrated optical detection device for an optical disk pickup disclosed in the paper of the Institution of Electronics Information and Communication Engineers of Japan published in 1986, Vol. 5, J69-C No. 5, P 609-P615. The conventional integrated optical pickup will be explained with reference to FIGS. 1 to 3.
FIG. 1 is a perspective view of the integrated optical pickup; FIG. 2A is a view of the integrated optical pickup of FIG. 1; and FIG. 2B is a view showing a state in which a semiconductor laser and an optical waveguide are coupled. On an Si substrate 51, there is formed an SiO.sub.2 buffer layer 58 having a refractive index of 1.46 to 1.47. Glass having refractive index of 1.55 is deposited on the surface of the buffer layer 58, whereby an optical waveguide 52 is formed. The optical waveguide 52 is shaped in a form of a thin film having a thickness of about 1 micron.
On the optical waveguide 52, there are formed a beam splitter 56 and a grating coupler 54 in its rear end side. Light receiving elements 57a, 57b and 57c, 57d which are respectively divided into two, are formed on both right and left sides of the front end portion on the Si substrate 51. In the front edge portion of the optical waveguide 52, there is arranged a semiconductor laser 53, which is fixed to a support member 53b. A thickness of an active layer 53a, which emits a laser beam of the semiconductor laser 53, is thinner than that of the optical waveguide 52, that is, about 0.1 micron.
The graph of FIG. 3 shows a coupling efficiency between conventional optical waveguides which are arranged to face each other. In this FIGURE, .LAMBDA. shows a shift distance in the upper and lower directions between the optical waveguides, and La shows a shift distance in the contact direction of the contact surfaces between the optical waveguides. From this graph, it can be understood that a suitable coupling efficiency can be obtained when .LAMBDA.=0 .mu.m and La=0 .mu.m. This result can be applied to a case in which the laser beam is incident into the optical waveguide from the active layer of the semiconductor laser.
The function of the integrated optical pickup will now be briefly explained.
The laser beam emitted from the semiconductor laser 53 is incident into the optical waveguide 52 from the end surface thereof, and transmitted through the optical waveguide 52 as the total reflection is repeated on the boundary surface between an air layer 9 and the buffer layer 58. Then, the laser beam is focused on the optical recording medium 5 through the beam splitter 56 and the grating coupler 54. The reflected light sent from the optical recording medium 5 is converged to the light receiving elements 57a, 57b and 57c, 57d as the total reflection is again repeated in the optical waveguide 52 through the grating coupler 54 and the beam splitter 56. Then, a focusing control and a tracking control of the integrated optical pickup are performed based on the signal detected by the light receiving elements 57a to 57d.
As mentioned above, there is proposed a technique in which the optical pickup is integrated and miniaturized, thereby reducing the weight. However, the above-mentioned technique has the following disadvantages.
Since the incidence of the laser beam from the semiconductor laser to the optical waveguide is performed on the end surface of the optical waveguide, the coupling efficiency largely depends on the state of the end surfaces. As mentioned above, since the thickness of the optical waveguide and that of the active layer of the semiconductor laser are 1 micron and 0.1 micron, respectively, it is difficult to adjust the optical waveguide and the active layer to be correctly coincident with each other in the upper and lower directions. Moreover, regarding the contact direction of the contact surfaces between the active layer and the optical waveguide, there is slightly generated a difference in assembling. Such a difference between the active layer and the optical waveguide generates a large change in the coupling efficiency as shown in the graph of FIG. 3. Therefore, if the coupling efficiency changes, the light emitted from the grating naturally changes.
In general, recording and reproducing characteristics in the optical recording medium largely depend on the quantity of light to be emitted to the recording medium. Due to this, it is required that the quantity of light to be emitted to the recording medium has sufficient density. Therefore, if there is generated a slight difference in the coupling state between the active layer of the semiconductor laser and the optical waveguide, the quantity of light, which is emitted from the grating to the recording medium, becomes insufficient, and an unfavorable influence is exerted on the recording and reproducing characteristics. Due to this, in the conventional technique, there is required extremely strict positioning accuracy between the semiconductor laser and the optical waveguide.
Moreover, it is known that the quantity of light emission of the semiconductor laser changes responsive to temperature changes and the passage of time. Due to the change of the quantity of light emission of the semiconductor laser, the quantity of light of the laser beam from the grating to the recording medium becomes unstable, so that an unfavorable influence is exerted on the recording and reproducing characteristics. In the above-mentioned conventional technique, there is no disclosure of any measures to be taken against the change of the quantity of light of beam.